Gaskets materials and applications
A flat gasket is a static sealing element placed between two contact surfaces, typically flanges, to prevent fluid or gas leaks when compressed by the clamping force of the bolts. The gasket fills surface irregularities and maintains the seal under pressure, temperature, and chemical attack. Its effectiveness depends on the material’s ability to deform and recover elastically, ensuring intimate and continuous contact.
Materials and properties
Section titled “Materials and properties”The selection of the flat gasket material is made based on temperature, pressure, and chemical nature of the fluid being sealed. The following table summarizes the operating limits of the most common materials used in the manufacture of flat gaskets.
| Gasket Material | Typical Fluids | Maximum Temperature | Maximum Pressure | Maximum Temperature × Pressure Product |
|---|---|---|---|---|
| Synthetic rubbers (NBR, EPDM, SBR) | Water, air | 121 °C / 250 °F | 4 bar / 60 psi | 496 °C·bar / 15 000 °F·psi |
| Vegetable fiber | Oil | 121 °C / 250 °F | 11 bar / 160 psi | 1 322 °C·bar / 40 000 °F·psi |
| Synthetic rubber with textile reinforcement | Water, air | 121 °C / 250 °F | 34 bar / 500 psi | 4 130 °C·bar / 125 000 °F·psi |
| Solid PTFE (Teflon) | Chemicals | 260 °C / 500 °F | 20 bar / 300 psi | 5 320 °C·bar / 150 000 °F·psi |
| Carbon steel | High-pressure fluids | 399 °C / 750 °F | 145 bar / 2 133 psi | 58 036 °C·bar / 1 600 000 °F·psi |
| Stainless steel | High-pressure or corrosive fluids | 649 °C / 1 200 °F | 171 bar / 2 500 psi | 110 636 °C·bar / 3 000 000 °F·psi |
| Spiral wound AISI304/PTFE | Chemicals | 260 °C / 500 °F | 34 bar / 500 psi | 8 866 °C·bar / 250 000 °F·psi |
| Spiral wound AISI304/Ceramic | Hot gases | 1 038 °C / 1 900 °F | 9 bar / 132 psi | 9 313 °C·bar / 250 000 °F·psi |
Synthetic rubbers include nitrile (NBR), EPDM, SBR and other elastomers, selected according to chemical resistance. Solid PTFE offers excellent chemical compatibility but low resilience; it is often used in flat form or as filler in spiral wound gaskets. Solid metallic gaskets (carbon or stainless steel) are used on flanges with machined seating and are typical of ring joint gaskets (RTJ). Spiral wound gaskets combine a spiral-wound metal strip with a filler material, offering elastic behavior and high sealing capacity.
Selection by application
Section titled “Selection by application”The following table provides a guideline for selecting the type of flat gasket based on the process fluid and service conditions.
| Application / Fluid | Recommended Material | Temperature Limit | Pressure Limit | Remarks |
|---|---|---|---|---|
| Hot water, low-pressure steam | Synthetic rubber with textile reinforcement | 121 °C / 250 °F | 34 bar / 500 psi | Good flexibility, economical |
| Mineral oils up to 120 °C | Vegetable fiber | 121 °C / 250 °F | 11 bar / 160 psi | Do not use with water or steam |
| Weak acids, dilute bases | Solid PTFE | 260 °C / 500 °F | 20 bar / 300 psi | Chemically inert, requires high clamping load |
| Hydrocarbons, steam up to 400 °C | Carbon steel (flat metal gasket or RTJ) | 399 °C / 750 °F | 145 bar / 2 133 psi | Requires good flange surface finish |
| Corrosive fluids at high temperature and pressure | Stainless steel (RTJ or flat) | 649 °C / 1 200 °F | 171 bar / 2 500 psi | Use material compatible with the medium (304, 316) |
| Severe chemical service, moderate pressure | Spiral wound AISI304/PTFE | 260 °C / 500 °F | 34 bar / 500 psi | Excellent elastic recovery and chemical resistance |
| Exhaust gases, turbines | Spiral wound AISI304/Ceramic | 1 038 °C / 1 900 °F | 9 bar / 132 psi | Withstands the highest temperatures |
For conditions that combine high temperature and high pressure simultaneously, the Temperature × Pressure product criterion from the previous section must also be met. In systems with thermal cycles or vibrations, spiral wound gaskets often perform better due to their elastic recovery capability.
Mounting considerations
Section titled “Mounting considerations”- Clean the bearing surfaces of the flanges; they must be free of scratches, rust, or remnants of previous gaskets.
- Check parallelism and flatness of the faces; excessive deviation prevents uniform seating.
- Select the gasket with correct inside and outside diameters according to the flange standard (ASME, DIN, etc.). For ring joint (RTJ) gaskets, the ring must have a hardness lower than that of the flange.
- Apply adequate compressive force. The minimum seating stress for non-metallic gaskets typically ranges between 10 MPa and 40 MPa (1 450 to 5 800 psi), while for metallic gaskets it can reach higher values depending on the material. The clamping pressure must be uniform, following a cross-tightening sequence and tightening progressively in several passes until the specified tightening torque is reached.
- Do not reuse gaskets unless expressly permitted by the manufacturer. Deformed gaskets lose their sealing capacity.
- Store gaskets in a dry place, away from heat sources and protected from UV light, especially those with an elastomeric base.
- On raised face flanges, the gasket must always seat within the machined contact surface, without invading the bolt holes.
Standardized gaskets and dimensions
Section titled “Standardized gaskets and dimensions”Flat gaskets used on standard flanges follow the dimensions of ASME B16.20 (spiral wound and ring joint gaskets) and ASME B16.5/B16.47 (flange dimensions). The following shows typical dimensions of spiral wound gaskets with an outer centering ring, according to ASME B16.20 Class 150.
| Nominal Pipe Size | Inside Diameter (ID) | Outside Diameter (OD) | Sealing Element Width | Total Thickness |
|---|---|---|---|---|
| 1/2“ (DN 15) | 3/4“ / 19.1 mm | 1 7/8“ / 47.6 mm | 0.175“ / 4.45 mm | 0.175“ / 4.45 mm |
| 1“ (DN 25) | 1 1/4“ / 31.8 mm | 2 5/8“ / 66.7 mm | 0.175“ / 4.45 mm | 0.175“ / 4.45 mm |
| 2“ (DN 50) | 2 3/4“ / 69.9 mm | 4 1/8“ / 104.8 mm | 0.175“ / 4.45 mm | 0.175“ / 4.45 mm |
| 4“ (DN 100) | 5“ / 127.0 mm | 6 7/8“ / 174.6 mm | 0.175“ / 4.45 mm | 0.175“ / 4.45 mm |
| 6“ (DN 150) | 7 3/16“ / 182.6 mm | 8 3/4“ / 222.3 mm | 0.175“ / 4.45 mm | 0.175“ / 4.45 mm |
| 8“ (DN 200) | 9 3/16“ / 233.4 mm | 11“ / 279.4 mm | 0.175“ / 4.45 mm | 0.175“ / 4.45 mm |
| 10“ (DN 250) | 11 5/16“ / 287.3 mm | 13 3/8“ / 339.7 mm | 0.175“ / 4.45 mm | 0.175“ / 4.45 mm |
RTJ (Ring Type Joint) gaskets are manufactured in carbon or stainless steel and are designated by a ring number (e.g., R-20, R-24) and an octagonal or oval profile. Their dimensions and tolerances are defined in ASME B16.20 and API 6A. They are used on flanges with machined grooves and are capable of sealing pressures up to 1 380 bar (20 000 psi) in high-pressure configurations.
Applicable standards
Section titled “Applicable standards”- ASME B16.20: Metallic gaskets for pipe flanges, including spiral wound and RTJ rings.
- ASME B16.5: Dimensions of steel flanges from 1/2“ to 24“ (Classes 150 to 2 500).
- ASME B16.47: Large diameter flanges (Series A and B).
- EN 1514: European series for flat gasket dimensions.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the maximum temperature that a flexible graphite gasket can withstand?
Section titled “What is the maximum temperature that a flexible graphite gasket can withstand?”Flexible graphite gaskets without oxidizers can operate up to 450 °C / 842 °F in a reducing or inert atmosphere, and up to 300 °C / 572 °F in the presence of air.
What tightening torque should be applied to a 4“ flange with a spiral wound gasket?
Section titled “What tightening torque should be applied to a 4“ flange with a spiral wound gasket?”For a 4“ ASME Class 150 flange with carbon steel bolts, the typical recommended torque is 120 N·m to 160 N·m (89 to 118 lb·ft), always following a cross-tightening sequence.
When is an inner ring mandatory for a spiral wound gasket?
Section titled “When is an inner ring mandatory for a spiral wound gasket?”ASME B16.20 requires an inner ring when the nominal pipe size is 10“ or larger, when the pressure class is 900 or higher, or when the flange inside diameter is more than 3 mm / 0.12 in larger than the gasket inside diameter.
What surface roughness should the flange faces have for a metal gasket?
Section titled “What surface roughness should the flange faces have for a metal gasket?”For RTJ gaskets, the groove roughness must not exceed 3.2 µm Ra / 126 µin Ra. For flat metal gaskets, a finish of 1.6 to 3.2 µm Ra (63 to 126 µin Ra) is recommended.
Can a spiral wound gasket be reused after being compressed?
Section titled “Can a spiral wound gasket be reused after being compressed?”No. Spiral wound gaskets deform plastically during the first tightening and lose their elastic recovery capacity. Attempting to reuse them can result in leaks because the necessary seating stress cannot be achieved.
What is the minimum width a flat gasket should have to seal properly?
Section titled “What is the minimum width a flat gasket should have to seal properly?”A minimum gasket width of 3 mm / 0.118 in is recommended for pressures below 40 bar / 580 psi. For higher pressures, the width is typically between 4 and 6 mm (0.157 to 0.236 in).
References
Section titled “References”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/gaskets-properties-d_34.html
- mcmaster.com: https://www.mcmaster.com/products/spiral-wound-gaskets/